The present invention relates in general to telecommunication line protection equipment, and more particularly to fail-safe circuits and apparatus for protecting telephone lines.
The transmission of information over the national telecommunications network involves the use of standard telephone subscriber lines and the like. One or more telephone lines extend from centralized switching systems directly to the respective telephone set, computer, answering machine, etc. in the subscriber""s home, office or building. As such, that portion of the telephone line that extends outside of a building structure is exposed to the environment. Because of this environmental exposure, there exists the possibility that aerial telephone lines can be either struck by lightning or crossed with power transmission lines. Buried telephone lines can also be subjected to hazardous voltages due to excavation, trenching or digging, where equipment can cut through and cause short circuits between buried power lines and the telephone lines. It can be appreciated that overvoltages caused by lightning and power line crosses can cause catastrophic damage, not only to the telephone lines themselves, but also to the equipment connected to the telephone lines.
As a result of the foregoing hazardous conditions, specifications and standards have been established for telephone line protection devices and circuits so that if such lines come in contact with hazardous voltages, the likelihood of damage to electrical equipment connected thereto is substantially reduced. Indeed, the standard telephone line protection module includes five terminals or pins that are arranged in a particular configuration. One pair of pins is associated with the telephone tip conductor and another pair is associated with a telephone line ring conductor. The fifth terminal is connected to a ground bus. One or more overvoltage protection circuits or devices are responsive to hazardous voltages to connect either the tip or ring conductors, or both, to the ground terminal so that the apparatus connected to the telephone line is protected from exposure to the hazardous voltages or currents.
U.S. Pat. Nos. 5,101,317; 5,341,270; and 5,359,657 are illustrative of conventional telephone line protection devices. In many of the telephone line protection devices, including those identified in the above-noted patents, primary overvoltage protection is provided by fast-acting solid state devices, many of which are obtained from Teccor Electronics, Inc. of Irving, Tex. These solid state devices are often referred to as xe2x80x9cSIDACtor(copyright)xe2x80x9d devices, and are described more fully in U.S. Pat. No. 5,479,031 which is assigned to Teccor Electronics, Inc. Secondary or fail-safe protection is often provided by other mechanical apparatus that is part of the telephone line protection module. The fail-safe apparatus is responsive to the thermal energy generated as a result of the overvoltage condition imposed on the telephone line conductor(s). This apparatus generally involves a material having a low-temperature melting point, such as a tin-lead solder which, when subjected to a temperature that causes melting thereof, enables a spring to force a conductor bar to move and short circuit the tip and ring line conductors to the ground terminal of the module. The melting point of the solder compounds generally used may be as low as 150xc2x0 F. Low-temperature melting solders are desirable because they trigger the fail-safe mechanism before extremely large currents destroy other parts or components connected to the telephone line. However, the low-temperature solder can in some instances melt due to elevated temperatures in contained cabinets and enclosures, in absence of any overvoltage being present on the telephone line conductors. This inadvertent activation is obviously undesirable.
The basic primary overvoltage protection and the fail-safe mechanism can be fabricated in a host of different variations, many of which are extremely complicated and thus expensive. Aside from the cost considerations of a mechanically complex device, the reliability is often compromised as a result of numerous mechanical and electrical components. It can be appreciated that as the number of components increase and the component complexity increases, the assembly time and cost also increase.
From the foregoing, it can be seen that a new type of telephone line protection device is needed which is designed to facilitate assembly thereof. Another need exists for an improved telephone line protection device that substantially reduces the number of components without compromising the reliability or quality thereof. Yet another need exists for a new telephone line protection device in which the protection element itself can be substantially assembled as an integral unit itself, and then attached to the 5-pin base so that the cover can be snap-locked thereto to form a module.
In accordance with the principles and concepts of the invention, an improved telephone line protection device is disclosed, of the type that overcomes or substantially reduces many of the shortcomings and problems attendant with the corresponding prior art devices. In accordance with a preferred embodiment of the invention, the protection element can be constructed using conventional semiconductor assembly line techniques. The protection element is constructed using three lead frames, each of which may be prestamped out of a spring-like conductive material, much like lead frames utilized in making multi-terminal semiconductor devices. When joined together by welding, or the like, the lead frames have spring-biased arms for capturing or holding a semiconductor cell therebetween. The semiconductor cell has contact surfaces that are electrically connected via the spring arms to the lead frames. In the preferred form, the semiconductor cell includes a SIDACtor circuit as well as diodes for providing current-sharing and steering capabilities. When fully assembled as an integral unit, the bottom lead frame of the protection element has five planar contact surfaces for either spot welding or fixed connection to the five pins of the telephone protection module. With this arrangement, very few components are required and very little manual labor is necessary in order to fully assemble the element and fix the same to the module pins.
In accordance with another embodiment, a fail-safe mechanism is employed. A semiconductor material of a specified resistivity is sandwiched between two of the lead frames by way of a high-temperature solder composition. One section of one of the lead frames is soldered to but spring-biased away from the semiconductor material. With this arrangement, when a substantial current passes through both lead frames via the resistive semiconductor material, a sufficient amount of heat is generated therein to melt the solder and cause the lead frame section to move under the influence of the spring bias away from the semiconductor material. The spring arm of the lead frame section then contacts another lead frame member, thereby providing a mechanical short circuit from one telephone line conductor to the ground terminal via the lead frame structure. Preferably, both the tip and ring lines employ separate fail-safe mechanisms so that both lines are independently protected from large currents that are considered more than transients.
As an alternative embodiment, the lead frame section having the spring biased arms can be split into two fingers so that only one finger is spring-biased while the other finger remains in contact with the semiconductor material, even after the solder has melted. With this arrangement, the tip or ring line is not open circuited to the equipment side of the protection module, but such line is nevertheless connected to ground by way of the other spring-biased finger.